Abstract
Understanding and controlling the (electro) chemical reactions between positive electrodes and electrolytes is essential to enhance the cycle life and safety of Li-ion batteries. Previous computational and experimental studies have shown that greater capacity loss of LiNixMnyCo1−x−yO2 (NMC) with increased Ni content can be attributed to the enhanced chemical oxidation of carbonate solvents by dehydrogenation and increased salt decomposition. In this study, we examine the role of a diphenyl carbonate (DPC) additive on the interfacial reactivity of LiNi1/3Mn1/3Co1/3O2, LiNi0.6Mn0.2Co0.2O2, LiNi0.8Mn0.1Co0.1O2 (NMC111, NMC622 and NMC811). Diffuse reflectance infrared Fourier Transform (DRIFT) spectroscopy on NMCs showed that adding DPC in the electrolyte suppressed signals associated with dehydrogenation of ethylene carbonate (EC) from LiNi1/3Mn1/3Ni1/3O2 to LiNi0.8Mn0.1Ni0.1O2 (NMC111 to NMC811). In addition, having DPC in the electrolyte was accompanied with less PF6− salt anion decomposition to form less-fluorine coordinated species such as lithium nickel oxyfluorides or PF3O-like species as revealed by combined infrared spectroscopy and X-ray Photoelectron Spectroscopy (XPS) for Ni-rich NMCs. Such observations are in agreement with previous work showing that DPC can increase the cycling performance of NMC811. The reduced reactivity between NMC such as NMC811 and electrolyte with DPC can be attributed to the formation of surface reaction products from the electrochemical oxidation of DPC occurring at lower voltages compared to the chemical oxidative dehydrogenation of carbonates. This hypothesis is supported by in situ infrared spectroscopy measurements, which revealed electrochemical oxidation of diphenyl carbonate upon charging at 3.9 VLi, accompanied by the detection of a feature around 1824 cm−1 attributed to organic oxidation products adsorbed on the oxide surface, and a stable electrode/electrolyte interface on NMC811 at higher voltages.
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